Web slitting assemblies are designed to cut continuously running webs in the longitudinal direction. They primarily consist of a blade and a band that contact each other axially at their periphery. The web is drawn through the intersection of the blade and band where it is severed longitudinally. The band is usually but not always driven a few percent faster than the web. The mating edges of the blade and band are ground at various angles to create sharp edges that shear the web.
To accomplish the shear action, the blade and band must be loaded axially against each other. In other words, the band is circular and has a side, and the blade is pressed against the side of the band. The nominal magnitude of the loading will vary depending upon the web product being cut. The precision of the loading will significantly affect the quality of the cut, and the life of the cutting edges of the blade and band.
Traditionally, the means of accomplishing the axial movement required to load a circular blade against a circular band has been to have the blade's axle sliding axially within a bushing. Another method used to a limited extent has been guiding the blade axially by means of a “4 bar linkage”. Each of these methods has an inherent drawback. In the case of the “axle and bushing” type of guiding, binding and friction will result in an inconsistent and undetermined load between the blade and the band.
A resisting force, theoretically equal to the designed applied force, is exerted by the band upon the blade at its periphery. This action presents a moment at the blade center that must be resisted by the axle within the bushing. The axle is required to move axially within the bushing while operating, due to minute run out that exists in the band throughout its rotation. Because of envelope restrictions, the ratio of the length of the bushing to the diameter of the axle (known as the L/D ratio) is relatively small. The aforementioned moment causes the axle-blade assembly to skew the axial axis to the extent of whatever clearance may exist in the axle bushing fit. This skewing results in the axial motion binding and therefore causing the intended loading to increase dramatically. Blade damage and wear result. This same phenomenon will occur, to a lesser extent, when a linear shaft bearing is used in place of the bushing referred to above.
In the case of the “4 Bar Linkage” type of guiding, envelope restrictions require that the pivots of the linkages be excessively small. This miniaturization requirement also essentially precludes the ability to include wear resistant elements, such as bearings or bushings, in the pivot design. Although this design, to a large degree, eliminates the binding aspect described for the axel-bushing arrangement, it does suffer from premature wear problems at the pivot points. Clearance in the pivots, even a small, required design clearance, will cause the blade assembly to tip out of the intended plane, that plane being essentially parallel to the face of the band. This compromise in alignment geometry results in a degradation of cut quality and blade and band life.
The clearance described, which increases with age, also allows the blade to move in response to forces generated by the shearing action. This will limit the cutting performance when encountering heavier web products that require higher cutting forces.
It would therefore be beneficial if there were a means of guiding the blade assembly in an axial direction without any resulting binding or friction. It would also be beneficial if the geometry of the blade with respect to the band would not degrade over time.